1. Field of the Invention
The present invention relates to an image pickup apparatus capable of performing focus control during a continuous shooting interval.
2. Description of the Related Art
A focus control apparatus used in electronic cameras is known that performs focus control by a contrast detection auto focus method. In the contrast detection auto focus method, the focus control apparatus determines the lens position that maximizes the high-frequency component of a luminance signal of a subject image formed on an image pickup device to be a focused focal point. Hereinafter, the high-frequency component is referred to as a “focus evaluation value”.
In addition, a focus control apparatus is known that performs focus control on the basis of a passive and active auto focus (AF). In the passive and active AF, the focus control apparatus receives a light beam reflected off a subject using a lens other than the imaging-capturing lens and a dedicated sensor. In addition, some focus control apparatuses employ the two above-described techniques and selects one of the two techniques as needed.
In recent years, many electronic cameras have employed the contrast detection auto focus method for a focus control apparatus. However, unlike a method of using a dedicated AF sensor, this method requires a scan operation of a focus lens in a light axis direction thereof, and therefore, additional time is generally required before focusing is achieved. Accordingly, in a mode that needs to reduce the shooting lag (such as in a continuous shooting mode), focus control is performed only once before starting the continuous shooting. In the subsequent continuous shooting operation, focus control is not performed. As such, Japanese Patent Laid-Open No. 2001-255456 describes a technique of reducing the increase in the time lag by switching between a focus control apparatus based on the contrast detection method and a focus control apparatus based on the passive/active AF using a dedicated sensor.
However, if an electronic camera includes a dedicated sensor for realizing the passive or active AF in addition to the focus control apparatus based on the contrast detection method, the body size and the fabrication cost of the electronic camera are increased. Therefore, it is difficult to apply this technology to all the electronic cameras.
The known technology for using a focus control apparatus based on the contrast detection method has the following disadvantages. For example, if the focus is locked at the focus lens position for the first image capturing operation in a continuous shooting mode and a subject moves beyond the depth of field during a continuous shooting operation, all the subsequent captured images are out of focus. In contrast, if the focus is adjusted by the contrast detection method every time an image is captured during a continuous shooting operation, the time interval between the image capturing operations is increased.